Editing Polystyrene (section)

==== Impact-resistant polystyrene (PS-I) ====
PS-I ('''''<u>i</u>'''mpact resistant '''<u>p</u>'''oly'''<u>s</u>'''tyrene'') consists of a continuous polystyrene matrix and a rubber phase dispersed therein. It is produced by polymerization of styrene in the presence of polybutadiene dissolved (in styrene). Polymerization takes place simultaneously in two ways:<ref>{{cite web|title=Schlagzähes PS auf chemgapedia.de|url=http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/10/styrol_polymerisation/schlagzaehes_ps/schlagzaehes_ps.vlu.html}}</ref>

* [[Graft polymerization|Graft copolymerization]]: The growing polystyrene chain reacts with a [[double bond]] of the [[polybutadiene]]. As a result, several polystyrene chains are attached to one polybutadiene. 
** <span style="color:#F46C2C">S</span> represents in the figure the styrene [[repeat unit]]
** <span style="color:#00AAC5">B</span> the butadiene repeat unit. However, the middle block often does not consist of such depicted butane homo-polymer but of a styrene-butadiene co-polymer:

:::<span style="color:#F46C2C">SSSSSS&shy;SSSSSSS&shy;SSSSSS</span><span style="color:#00AAC5">BB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">BB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span>&shy;<span style="color:#F46C2C">S</span><span style="color:#00AAC5">BBBB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span>&shy;<span style="color:#F46C2C">SS</span><span style="color:#00AAC5">BBB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span><span style="color:#F46C2C">SSSSSSS&shy;SSSSSSS&shy;SSSSSSSSSSSSSSS</span><span style="color:#F46C2C">S</span>

By using a statistical copolymer at this position, the polymer becomes less susceptible to [[Cross linking|cross-linking]] and [[Melt flow index|flows]] better in the melt. For the production of SBS, the first styrene is homopolymerized via anionic copolymerization. Typically, an organometallic compound such as butyllithium is used as a catalyst. Butadiene is then added and after styrene again its polymerization. The catalyst remains active during the whole process (for which the used chemicals must be of high purity). The [[molecular weight distribution]] of the polymers is very low ([[polydispersity]] in the range of 1.05, the individual chains have thus very similar lengths). The length of the individual blocks can be adjusted by the ratio of catalyst to monomer. The size of the rubber sections, in turn, depends on the block length. The production of small structures (smaller than the wavelength of the light) ensure transparency. In contrast to PS-I, however, the block copolymer does not form any particles but has a lamellar structure.